Roger Weller, geology instructor
|1. conglomerate||2. breccia||3. sandstone||4. siltstone|
|5. shale||6. limestone||7. dolomite||8. gypsum|
|9. salt||10. chert||11. coal||12. amber|
Sedimentary rocks fall into three major categories:
clastic sedimentary rocks chemical sedimentary rocks biological sedimentary rocks
Rock Photos for
visual examples of each of these
sedimentary rock types.
Clastic (Fragmental) Sedimentary Rocks
Conglomerate is made of rounded or semi-rounded rock fragments cemented
together. The rounding of the fragments implies that the fragments were transported
a substantial distance from their source and were abraded in contact with other
moving fragments. The rounded fragments were probably deposited along a stream
channel or a shoreline. Fragments within a conglomerate are pea-sized and larger.
An older name for conglomerate is "pudding stone".
Breccia consists of angular rock fragments cemented together. The angular shape
implies that the fragments have not moved far from their source. Fragments are
pea-sized and larger, similar to conglomerate. Commonly, breccias are found along
fault zones. Breccias can be any color.
Sandstone consists of sand grains that have been cemented together. Sandstones
can range from coarse-grained to fine-grained. You should be able to distinguish the
sand grains with the naked eye. Light-colored sandstones consisting mainly of
rounded, well-sorted, quartz grains are referred to as mature sandstones or
quartz sandstones. Sandstones that contain angular grains of several different
minerals are referred to as immature sandstones or graywackes. Sandstones
containing feldspar grains are arkoses. Sandstones can be white, gray, pink, red,
brown, or black. A fresh broken surface of sandstone has a gritty feel.
Siltstone is made of silt-sized particles, finer than sand grains, but coarser than
clay. It is a difficult rock to identify because it closely resembles a fine-grained
sandstone or a coarse shale. Then general way of describing it is that you cannot
see the individual grains, but the surface of siltstone has a slightly feel to it.
Siltstones occur in a wide range of colors.
Shale is made of clay-sized particles or clay minerals that have compressed by
the weight of overlying rocks. Generally, shale has the tendency to split in fairly
flat fragments; this property is known as fissility. Shales can be many colors, such
as black, gray, brown, red or gray, depending upon the presence of organic
materials and iron oxides. Shales represent the accumulation of clay at the
bottom of oceans or lakes. Shales are often a good source of fossils.
Chemical Sedimentary Rocks
There are dozens of forms that limestone can take, often making it visually
difficult to identify. However, since limestone is made of the mineral calcite
(CaCO3), it will bubble freely when strong hydrochloric acid is applied to it.
Limestone varies from light gray or brown to dark gray or brown.
Common forms of limestone include:
coquina limestone made of broken shell fragments
fossiliferrous limestone rich in fossils
lithographic limestone very fine-grained and dense
chalk fine-grained porous
encrinal limestone made of crinoid fragments
travertine deposited by surface waters (noted for its holes)
Dolomite looks almost exactly like calcite. There is a good reason for this
similarity. Dolomite originally started out as limestone but was chemically altered
at a later time by the replacement of some of its calcium by magnesium. When
hydrochloric acid is applied to dolomite, it fizzes (produces bubbles) at a much
slower rate than limestone (calcite).
Gypsum is the name of both the mineral and rock, although there have been
attempts to separate the two by calling the rock gypstone. Gypsum is softer than
your fingernail and can be scratched or bruised easily. It does not taste like salt
and it does not fizz when hydrochloric acid is applied to it. Gypsum is usually white
or a pale reddish-brown when stained by iron oxide. Gypsum is baked at high
temperatures to drive water out of its chemical structure and then ground
to a fine powder to produce plaster of Paris.
Salt is the mineral halite (NaCl) that was deposited by the evaporation of a body
of salt water. Typically, salt is white or colorless, but it might also be lightly colored
by the inclusion of iron oxide or clay. Salt can easily be identified by its salty taste,
but it is generally not recommended to lick strange rocks. Salt is water soluble,
producing a melted-looking surface when it is washed off with water. It can also be
identified by the cubical cleavage of halite (salt).
Chert is chemically deposited cryptocrystalline quartz, usually a dull gray or
brown in color. It is commonly found as nodules embedded in limestone which
project out of the limestone as the limestone is slowly dissolved by rainwater.
If the chert has a waxy luster rather than a dull surface, it is called flint. Flint also
tends to chip with conchoidal fractures better than chert; it is this property that allows
flint to be made into arrowheads. Jasper is chert that is colored red,
reddish brown, or bright yellowish brown.
Biological Sedimentary Rocks
Coal represents the accumulation of decomposed plant materials. Coal is sorted
by the degree of alteration and compaction of the original organic materials. The
least altered material is peat, followed by lignite, then bituminous coal, and finally
anthracite. In peat, you can still see an abundance of the original organic materials.
Lignite is called soft, brown coal. Bituminous coal is black, and somewhat waxy-looking.
Anthracite is hard, black coal. Peat has the lowest level of carbon content and anthracite
has the highest. Coal is less dense than normal rocks.
Amber is ancient, hardened tree sap. It is a natural plastic and it is light-weight.
When you first pick it up, your first impression is that it is much lighter in weight than
a typical stone. Unworked amber has a dull surface marked by a myriad of
minute fractures. The clear, inner amber can only be exposed by chipping off a corner
of the stone or grinding of the surface. Amber ranges from a creamy yellow to
ransparent yellow or red to a dark brown. Amber is famous for preserving
trapped insects for millions of years.